In order to provide for efficient operation of steam turbines and to prevent blade fracturing, systems are known for determining the vibrations of shrouded turbine blades. These systems generate data associated with the position of the turbine blades at a specified time, as well as blade revolution data. This data is received by a monitor and blade vibration information may be extracted therefrom. Turbine blade vibration information is used for various purposes, including early detection of blade stresses which may cause the blade to fracture at some time in the future.
As described in U.S. Pat. No. 5,097,771, titled Shrouded Turbine Blade Vibration Monitor and Target Therefor, assigned to the assignee of the present invention, blade vibration monitoring systems generally comprise a stationary sensor located within a seal around the shrouded turbine blades. The sensor induces eddy currents in the shroud segment of the turbine blades as each shroud segment passes the sensor. A target is located on each shroud segment for causing a variation in the induced eddy currents. The sensor is responsive to the variation in the induced eddy currents to produce an output signal containing information corresponding to the time of arrival of the target at the sensor. Another sensor is responsive to indicia carried by the turbine rotor for producing a once per revolution pulse. A blade vibration monitor is responsive to the output signal and the once per rotor revolution pulse, and blade vibration information may be determined from this data.
Various forms of targets are known for providing the variations in the eddy currents induced by the stationary sensor used in a blade vibration monitoring system, as described in U.S. Pat. No. 5,097,711. These targets include notches, protrusions, conductive foils, conductive material deposited on the turbine shroud, shallow slots formed in the turbine shroud which are filled with a conductive material, and notches cut in the leading and/or trailing edges of the shroud segment.
However, it has been found that the use of these targets entails significant costs and, in some applications, may actually decrease the long term performance capabilities of the turbine shroud. In order to provide a notch target or, in the alternative, to deposit a highly conductive material directly onto the shroud, sophisticated equipment must be brought to the turbine site and must be capable of being operable within the confines of the turbine machinery. One of ordinary skill in the art would recognize the significant costs associated with the use of these targets. One of ordinary skill would also recognize that a notch in the shroud segment may cause concentrated stress at that location and may be a cite for crack initiation within the shroud.
Therefore there is a need for a target for use in a shrouded turbine blade monitoring system which will efficiently provide data associated with turbine blade vibrations, yet may be installed onto and removed from the shroud segment of the turbine blades at a low cost.
Accordingly it is an object of the present invention to provide a shrouded turbine blade monitoring target comprising a shim which is plated with a highly conductive material and which may be spot welded to the shroud segment of shrouded turbine blades in order to provide a signal which is adapted for monitoring turbine blade vibration.